1. Field of the Invention
This invention relates to apparatus for dispersing and dissolving a gas in a liquid and, in particular, an aerator adapted to ejecting atmospheric oxygen in the form of a bubble-containing stream and thereby dissolving it in ambient water, as well as a process for the aerobic biological treatment of, for example, organic waste water, various liquid media used in biotechnology, and lake water by using such an aerator.
2. Description of the Prior Art
Conventionally known apparatus for dissolving a gas in a liquid include, for example, an apparatus for dissolving atmospheric oxygen in water which comprises air diffuser pipes disposed in a water tank for blowing air through the water; an apparatus for dissolving atmospheric oxygen in water which comprise an ejecter mechanism for mixing air with water in advance and then ejecting the resulting mixture (as described, for example, in Japanese Utility Model No. 42560/'82 and Japanese Patent Publication No. 41290/'82).
However, when an apparatus of the air diffuser pipe type is used, relatively large air bubbles (with a diameter of ten to several tens of millimeters) are formed. For this reason, the dissolution efficiency of atmospheric oxygen (i.e., the proportion of the amount of oxygen dissolved to the amount of oxygen supplied) is so low that the maximum oxygen dissolution efficiency is as low as about 7% and, therefore, the water within the tank having air diffuser pipes disposed therein has a very low average oxygen concentration. On the other hand, an apparatus of the ejector type can discharge relatively minute air bubbles in the form of a horizontal stream. Accordingly, as compared with processes using air diffuser pipes or mechanical aeration, the oxygen dissolution efficiency is relatively high and air can be relatively, uniformly dispersed throughout the water being treated. However, conventional ejectors have the disadvantage that a stream containing minute air bubbles can be produced only when the mixing ratio of air to water is about 1/1 or lower. As the amount of air supplied is increased, the diameter of the resulting air bubbles becomes larger and these air bubbles become more liable to coalesce into even larger air bubbles, so that the dispersion of air bubbles in the water becomes very poor and the oxygen dissolution efficiency is reduced. For this reason, it has been impossible to supply a gas in sufficiently large amounts.